In the earliest prior art the method of locating the position of a remote transmitter was to utilize a direction finding (DF) triangulation technique where signals from the transmitter are received at widely spaced DF antenna sites. A line-of-bearing (LOB) measurement to the transmitter is measured at each antenna site. When the LOBs are plotted on a map they intersect at the transmitter location. The accuracy of this intersection is directly related to the accuracy of these lines-of-bearing.
In interferometric radio direction finding systems, the phase difference between signals received by two antennas and receivers is measured to determine the angle of arrival of the signal since, for a fixed separation distance between the two sensors, the phase difference is directly related to the angle to be measured. However, interferometric techniques to measure the angle of arrival of an RF signal have posed problems in implementation of the system and accuracy of the measurement.
Interferometers, even those having a long baseline, are subject to a reduced accuracy. The accuracy is greatly reduced by decorrelation of the signals at the two interferometric sensors due principally to the differences of scattering and multi-path propagation effects at the two sensors. In addition, they are frequency sensitive and suffer from phase ambiguity problems.
An alternate approach to determining the angle of arrival of a RF signal is to measure the “time difference of arrival” of the signal between two antennas/receivers that are spaced apart by a distance greater than one wavelength of the RF signal. This is known in the art as TDOA. It may be accomplished for any baseline length so long as the difference in arrival times can be accurately measured since, for a fixed baseline, the delay is directly related to the angle to be measured. Owing to the direction of propagation of the wavefront with relation to the two antennas the wavefront is initially received by a first antenna and subsequently received by the second antenna unless the bearing angle is equal to zero. The TDOA system measures the differential arrival time of the RF signal at the two antennas. For an example see FIG. 1. For each delay in time of arrival of a radio frequency wavefront at the two antennas, there is defined an isodelay hyperbolic line of position upon which the transmitter lies. However, each LOP only locates a transmitter to an arbitrary position on an LOP curve so multiple measurements are taken yielding multiple LOPs. Theoretically, in a perfect system when all hyperbolic lines of position are graphed they will intersect at a single point and the transmitter's location is estimated to be at the point of intersection. However, because of the presence of noise, this is not the case and the multiple isodelay hyperbolic lines of position do not intersect at a single point but, instead, define an “area” in which is located the transmitter. The smaller the “area” the better is the geolocation of the transmitter. TDOA geolocation systems are developed by placing the antennas and receivers on separate platforms. During the measurement periods, the geolocation of these platforms must be accurately known.
Another technique for transmitter position location is known as “frequency difference of arrival” (FDOA), also known in the art as Differential Doppler, and it measures the relative frequency difference (Doppler shift) of a signal arriving at the antennas of different RF antennas/receivers. As an FDOA RF receiver moves directly toward a transmitter the Doppler shift yields the highest positive frequency shift, and as the RF receiver moves directly away from a transmitter the Doppler shift yields the highest negative frequency shift. Movement toward or away from the transmitter at a different angle yields a frequency shift between these two extremes. An FDOA frequency difference measurement defines an isodoppler “line of position” (LOP) upon which the unknown transmitter lies. However, each LOP only locates a transmitter to an arbitrary position on an LOP curve. Due to noise, the FDOA LOPs from multiple antennas/receivers intersect do not intersect at a single point, but typically define an area that provides an estimate of the geolocation of the transmitter. For an example see FIG. 2.
Cross-correlation of a signal received at two antennas/receivers is a well known and commonly used signal processing technique for measuring the relative time delay (TDOA) and frequency shift (FDOA) of a signal. The noise associated with the received signals distorts the cross-correlation result, thereby degrading the accuracy of the TDOA and FDOA measurements. When both techniques are combined the isodelay and isodoppler lines of position (LOPs), jointly define the area in which the transmitter is located. The accuracy of these locations is taught in a paper by Paul C. Chestnut, “Emitter Location Accuracy Using TDOA and Differential Doppler,” IEEE Trans. on Aerospace and Electronic Systems, Vol. AES-18, No. 2, pp. 214-218 March 1982
Because of noise and interfering signals, the combined TDOA/FDOA signal prevents the multiple isodelay and isodoppler lines of position (LOPs) from intersecting at the same point. Rather, the lines intersect at several points, as shown in FIG. 3, forming an area or region within which the transmitter should be located, rather than defining a precise position location.
At times, the TDOA estimates constituting the system of equations contain redundancies which make the system solutions inconsistent. In this case there is no unique solution, and the fix estimate can only be optimized according to some error criterion, such as the least-squares error. Even then there are still significant errors. Several methods have been proposed for inconsistent systems. One is the Spherical Intersection Method taught in a paper by H. C. Schau and A. Z. Robinson, “Passive Source Localization Employing Intersection Spherical Surfaces for Time-of-Arrival Differences,” IEEE Trans. on Acoustics, Speech, and Signal Processing, Vol. ASSP-35, No. 8, August 1987, pp. 984-995. A second is the Spherical-Interpolation Method taught in a paper by B. Friedlander, “A Passive Localization Algorithm and Its Accuracy Analysis,” IEEE Journal of Oceanic Engineering, Vol. OE-12, No. 1, pp. 234-245, January 1987. A third is the Divide and Conquer Method taught in a paper by J. S. Abel, “A Divided-and-Conquer Approach to Least-Squares Estimation,” IEEE Trans. on Aerospace and Electronic Systems, Vol. 26, No. 2, pp. 423-427, March 1990.
Transmitter position location has also been achieved by “correlation interferometer geo-location” (CIGL) which requires fewer antennas and associated receivers than utilized in the prior art. Very broadly, this is accomplished by moving correlation processing from the direction finding AOA function into the transmitter location function, and it is based on the correlation summation of voltages measured at the antennas of a DF antenna array. One such CIGL system is taught in PCT Patent application No. US 2004/03373, filed Feb. 6, 2004, and entitled “Correlation Interferometer Geo-location”. Another such GIGL system is taught in U.S. patent application Ser. No. 11/249,922, filed Oct. 13, 2005, and entitled “Moving Transmitter Correlation Interferometer Geo-location”.
One of the features of TDOA/CIGL that makes it stand apart from the prior art is its ability to effortlessly integrate data from multiple platforms, and/or multiple collection episodes. Prior to the appearance of TDOA/CIGL, one could derive a TDOA estimate for each pair of antennas and receivers, and from that, construct a set of hyperbolas representing the mapping of TDOA onto 3-dimensional space. The difficult task then remained of solving a nonlinear system of equations that described the TDOA hyperbolas, in order to arrive at a position estimate. In those cases with an inconsistent system of equations, the task became one of finding the best position estimate in a least square error sense. A TDOA/CIGL algorithm obviates the need for solving these sets of nonlinear equations.
Thus, there is a need in the art for a way to combine the strengths of the TDOA and FDOA methods of transmitter location and the strengths of the CIGL processing technique to achieve more accurate results in geolocating transmitters.